Optical interference coatings, sometimes also referred to as thin film optical coatings or filters, comprise alternating layers of two or more materials of different indices of refraction. Some such coatings or films have been used to selectively reflect or transmit light radiation from various portions of the electromagnetic radiation spectrum, such as ultraviolet, visible and infrared radiation. For instance, optical interference coatings are commonly used in the lamp industry to coat reflectors and lamp envelopes. One application in which optical interference coatings are useful, is to improve the illumination efficiency or efficacy of lamps, by reflecting energy emitted (by, e.g., a filament or arc) back towards the filament or arc while transmitting visible light of the electromagnetic spectrum emitted by the light source. This decreases the amount of electrical energy necessary for the light source to maintain its operating temperature.
Optical interference coatings generally comprises two different types of alternating layers, one having a low refractive index and the other having a high refractive index. With these two materials having different indices of refraction, an optical interference coating, which can be deposited on the surface of the lamp envelope, can be designed. In some cases, the coating or filter transmits the light in the visible spectrum region emitted from the light source while it reflects the infrared light. The returned infrared light heats the light source during lamp operation and, as a result, the lumen output of a coated lamp is considerably greater than the lumen output of an uncoated lamp.
With the advent of potential energy regulations for incandescent and halogen lamps, it has become increasingly important to develop and introduce energy efficient products.